Patient monitoring of various physiological parameters of a patient is important to a wide range of medical applications. Oximetry is one of the techniques that has developed to accomplish the monitoring of some of these physiological characteristics. It was developed to study and to measure, among other things, the oxygen status of blood. Pulse oximetry—a noninvasive, widely accepted form of oximetry—relies on a sensor attached externally to a patient to output signals indicative of various physiological parameters, such as a patient's constituents and/or analytes, including for example a percent value for arterial oxygen saturation, carbon monoxide saturation, methemoglobin saturation, fractional saturations, total hematocrit, bilirubins, perfusion quality, or the like. A pulse oximetry system generally includes a patient monitor, a communications medium such as a cable, and/or a physiological sensor having light emitters and a detector, such as one or more LEDs and a photodetector. The sensor is attached to a tissue site, such as a finger, toe, ear lobe, nose, hand, foot, or other site having pulsatile blood flow which can be penetrated by light from the emitters. The detector is responsive to the emitted light after attenuation by pulsatile blood flowing in the tissue site. The detector outputs a detector signal to the monitor over the communication medium, which processes the signal to provide a numerical readout of physiological parameters such as oxygen saturation (SpO2) and/or pulse rate. The detector signal can also be used by the monitor to create an image on a display screen of the tissue being monitored.
High fidelity pulse oximeters capable of reading through motion induced noise are disclosed in U.S. Pat. Nos. 7,096,054, 6,813,511, 6,792,300, 6,770,028, 6,658,276, 6,157,850, 6,002,952 5,769,785, and 5,758,644, which are assigned to Masimo Corporation of Irvine, Calif. (“Masimo Corp.”) and are incorporated by reference herein. Advanced physiological monitoring systems can incorporate pulse oximetry in addition to advanced features for the calculation and display of other blood parameters, such as carboxyhemoglobin (HbCO), methemoglobin (HbMet), total hemoglobin (Hbt), total Hematocrit (Hct), oxygen concentrations, glucose concentrations, blood pressure, electrocardiogram data, temperature, and/or respiratory rate as a few examples. Typically, the physiological monitoring system provides a numerical readout of and/or waveform of the measured parameter. Advanced physiological monitors and multiple wavelength optical sensors capable of measuring parameters in addition to SpO2, such as HbCO, HbMet and/or Hbt are described in at least U.S. Pat. No. 7,764,982, and U.S. application Ser. No. 11/366,208, filed Mar. 1, 2006, titled Noninvasive Multi-Parameter Patient Monitor, assigned to Masimo Laboratories, Inc. and incorporated by reference herein. Further, noninvasive blood parameter monitors and optical sensors including Rainbow™ adhesive and reusable sensors and RAD-57™ and Radical-7™ monitors capable of measuring SpO2, pulse rate, perfusion index (PI), signal quality (SiQ), pulse variability index (PVI), HbCO and/or HbMet, among other parameters, are also commercially available from Masimo Corp.
During blood circulation, arteries carry blood away from the heart in high volume and under high pressure. Arteries branch off into smaller blood vessels, called arterioles. Arterioles are well innervated, surrounded by smooth muscle cells, and are about 10-100 μm in diameter. Arterioles carry the blood to the capillaries, which are the smallest blood vessels, which are not innervated, have no smooth muscle, and are about 5-8 μm in diameter. Blood flows out of the capillaries into the venules, which have little smooth muscle and are about 10-200 μm in diameter. The blood flows from venules into the veins, which carry blood back to the heart. Arterioles, venules, and/or capillaries may also be referred to as microvessels.
Microcirculation generally refers to the vascular network lying between the arterioles and the venules, including the capillaries, as well as the flow of blood through this network. These small vessels can be found in the vasculature which are embedded within organs and are responsible for the distribution of blood within tissues as opposed to larger vessels in the macrocirculation which transport blood to and from the organs. One of the functions of microcirculation is to deliver oxygen and other nutrients to tissue. Sometimes, microcirculation in these small vessels can become blocked, interfering with the delivery of oxygen to the tissue.